Tuesday, June 2, 2020

10:00 am (CST)

Ashvin Vishwanath (Harvard University)

Title: Charged Skyrmions and Superconductivity in Magic Angle Graphene (talk link)

Abstract: In bilayer graphene twisted near a magic angle, both interaction-driven insulators and superconductors have been experimentally realized on varying the electron filling. We propose a topological mechanism for the observed superconductivity, that ties the superconductor to the interaction-driven insulating phases. Specifically we show that a promising candidate for an ordered insulating phase has the property that skyrmions in its order parameter carry electric charge. Skyrmion condensation can then lead to a superconductor - whose properties we calculate within the theory. More generally we identify a physical setting which can promote a new mechanism for superconductivity, the pairing of charged topological textures, which can mitigate the effects of Coulomb repulsion.

Work done in collaboration with Eslam Khalaf, Shubhayu Chatterjee, Nick Bultinck, Shang Liu and Mike Zaletel. arXiv:2004.00638 and arXiv:1911.02045

Chair: Natalia Perkins

Tuesday, June 16, 2020

10:00 am (CST)

Cristian Batista (University of Tennessee)

Title: Real space Berry curvature of itinerant electron systems with spin-orbit interaction (talk link)

Abstract: By considering an extended double-exchange model with spin-orbit coupling (SOC), we derive a general form of the Berry phase γ that electrons pick up when moving around a closed loop. This form generalizes the well-known result valid for SU(2) invariant systems, γ = Ω/2, where Ω is the solid angle subtended by the local magnetic moments enclosed by the loop. The general form of γ demonstrates that collinear and coplanar magnetic textures can also induce a Berry phase different from 0 or π, smoothly connecting the result for SU(2) invariant systems with the well-known result of Karplus and Luttinger for collinear ferromagnets with finite SOC. By taking the continuum limit of the theory, we also derive the corresponding generalized form of the real space Berry curvature. The new expression is a generalization of the scalar spin chirality, which is presented in an explicitly covariant form. We finally show how these simple concepts can be used to understand the origin of the spontaneous topological Hall effect that has been recently reported in collinear and coplanar antiferromagnetic phases of correlated materials.

Work done in collaboration with Shang-Shun Zhang, Hiroaki Ishizuka, Hao Zhang, Gábor B. Halász. Phys. Rev. B 101, 024420 (2020).

Chair: Yong-Baek Kim

Tuesday, June 30, 2020

10:00 am (CST)

Louis Taillefer (University of Sherbrooke)

Title: New signatures of the pseudogap phase of cuprate superconductors (talk link)

Abstract:The pseudogap phase of cuprate superconductors is arguably the most enigmatic phase of quantum matter. With several collaborators, our aim has been to shed new light on this phase by investigating the non-superconducting ground state of cuprate materials at low temperature across a wide doping range, suppressing superconductivity with a magnetic field [1].Hall effect and thermal conductivity measurements across the pseudogap critical doping p* reveal a sharp drop in carrier density n from n = 1 + p above p* to n = p below p* [2,3,4], signaling a major transformation of the Fermi surface. Angle-dependent magneto-resistance (ADMR) directly reveals a change in Fermi surface topology across p* [5]. From specific heat measurements, we observe the classic thermodynamic signatures of quantum criticality: the electronic specific heat C_el shows a sharp peak at p*, where it varies in temperature as C_el ~ – T logT [6]. At p* and just above, the electrical resistivity is linear in T at low T, with an inelastic scattering rate that obeys the Planckian limit [7]. Finally, the pseudogap phase is found to have a large negative thermal Hall conductivity, which extends to zero doping [8]. We show that the pseudogap phase makes phonons become chiral [9].

[1] Proust & Taillefer, Annu. Rev. Condens. Matter Phys. 10, 409 (2019).

[2] Badoux et al., Nature 531, 210 (2016).

[3] Collignon et al., Phys. Rev. B 95, 224517 (2017).

[4] Michon et al., Phys. Rev. X 8, 041010 (2018).

[5] Fang, Grissonnanche et al., arXiv:2004.01725 (2020).

[6] Michon et al., Nature 567, 218 (2019).

[7] Legros et al., Nat. Phys. 15, 142 (2019).

[8] Grissonnanche et al., Nature 571, 376 (2019).

[9] Grissonnanche et al., arXiv:2003.00111 (2020).

Chair: James Analytis

Tuesday, July 14, 2020

10:00 am (CST)

Radu Coldea (Oxford University)

Title: Neutron scattering studies of touching points in magnon bands (talk link)

Abstract: Complementary to studies of symmetry-protected band-touching points for electron bands in metallic systems, we explore analogous physics for propagating bosonic quasiparticles, magnons and spin-orbit excitons, in the insulating easy-plane honeycomb quantum magnet CoTiO3. We probe directly the winding of the isospin texture of the quasiparticle wavefunction in momentum space near a nodal point through its characteristic fingerprint in the dynamical structure factor probed by inelastic neutron scattering. In addition, our high-resolution measurements reveal a finite spectral gap at low energies, which cannot be explained by a semiclassical treatment for the ground state pseudospins-1/2. As possible mechanisms for the spectral gap generation we propose quantum-order-by-disorder induced by bond-dependent anisotropic couplings such as Kitaev exchange, and higher-order spin-orbital exchanges. We provide a spin-orbital flavor-wave model that captures both the gapped magnons and dispersive spin-orbit excitons within the same Hamiltonian.

Work done in collaboration with M. Elliot, P.A. McClarty, D. Prabhakaran, R.D. Johnson, H.C. Walker, P. Manuel, arXiv:2007.04199

Chair: James Analytis

Tuesday, July 28, 2020

10:00 am (CST)

Yong-Baek Kim (University of Toronto)

Title: Non-Fermi Liquids in Multipolar Materials (talk link)

Abstract: The hallmarks of non-Fermi liquids are singular thermodynamic and transport properties that are distinct from those associated with a Fermi liquid. Non-Fermi liquid behaviors are famously seen in cuprates, heavy fermion materials, and metallic quantum critical systems. In this talk, I discuss possible non-Fermi liquids in multipolar materials, where conduction electrons interact with the local moments that do not carry any dipole moment, but possess higher-rank multipolar moments. This theoretical work is partly motivated by recent experiments on cubic f-electron systems, where the local moments arise from non-Kramers ground states. I present the renormalization-group and conformal-field-theory solution of a multipolar Kondo problem, where a single multipolar moment is interacting with the orbital and spin degrees of freedom of conduction electrons. I show that an unexpected non-Fermi liquid state emerges and discuss its broader implications to existing and future experiments. Work done in collaboration with Adarsh Patri, arXiv:2005.08973 and an earlier work with Adarsh Patri and Ilia Khait, arXiv:1904.02717.

Chair: Natalia Perkins

Tuesday, August 11, 2020

10:00 am (CST)

Siddharth A. Parameswaran (University of Oxford)

Title: Symmetry and topology of quasiparticles and their bound states in correlated insulators (talk link)

Abstract: In this talk, I explore two examples of how the symmetry of a microscopic lattice or topology of an underlying non-interacting band structure can be imprinted in the excitations of strongly-correlated insulators. First, I show that previously-unnoticed crystal symmetry constraints drastically alter the understanding of Ising quantum criticality in the quasi-1D magnetic insulator CoNb2O6, resolving decade-old puzzles related to the dispersion of confined `kinks’ in the ordered phase and the decay of spin-flip quasiparticles in the paramagnetic phase [1]. I will then change focus to the correlated quantum anomalous Hall insulator recently observed in twisted bilayer graphene near the magic angle, where I will discuss the topological properties [2] and many-body physics [3] of excitonic bound states.

References:

[1] M. Fava, R. Coldea, and S.A. Parameswaran, arXiv:2004.04169 (2020).

[2] Y.H. Kwan, Y. Hu, S.H. Simon, and S.A. Parameswaran, arXiv:2003.11560 (2020).

[3] Y.H. Kwan, Y. Hu, S.H. Simon, and S.A. Parameswaran, arXiv:2003.11559 (2020).

Chair: Yong-Baek Kim

Tuesday, August 25, 2020

10:00 am (CST)

Peter Armitage (The Johns Hopkins University)

Title: Novel Measure of Quantum Correlations Using Nonlinear THz Response (talk link)

Abstract: The rate of discovery in quantum correlated materials in recent years has been remarkable. From 2D materials, to topological insulators, to strong spin-orbit coupled systems, to quantized responses in spin-liquids and fractionalization, these advances challenge our previous notions of the possible behavior of electrons in solids. However, it is now clear that many of their most interesting purported properties remain hidden to us. They have distinct quantum mechanical correlations that are in many cases only indirectly accessible with current techniques. For instance, they can have Berry phase structures in momentum space, fractionalized quasiparticles, or many-body entanglement that we simply do not have the tools to characterize properly. There can also be new forms of classical order that are largely invisible to conventional scattering techniques. Going forward it is going to be essential to develop new techniques and instrumentation that can reveal their properties. One of the most promising directions to get qualitatively new information is the use of nonlinear optical spectroscopies and particularly those in the THz range. For a number of reasons nonlinear spectroscopic responses can give unique information about quantum correlations that are completely inaccessible at the level of linear response. In this talk I will discuss my group's efforts to develop and use nonlinear THz spectroscopic techniques to probe unique aspects of quantum materials. I will center on the new technique of THz 2D coherent spectroscopy from both a theoretical and experimental perspective. I will give examples of this technique's use to probe phenomena as diverse as fractionalization in spin liquids, “marginal quasiparticles” in electronic glasses on the insulating side of the 3D metal-insulator transition, and the strange metal state of cuprate superconductors.

[1] Wan, Y., & Armitage, N. P. (2019). "Resolving continua of fractional excitations by spinon echo in THz 2D coherent spectroscopy”, Physical review letters, 122(25), 257401.

[2] Mahmood, F., Chaudhuri, D., Gopalakrishnan, S., Nandkishore, R., & Armitage, N. P. (2020). "Observation of a marginal Fermi glass using THz 2D coherent spectroscopy," arXiv preprint arXiv:2005.10822.

Chair: James Analytis